The present invention relates to an injection molding method preferably applied to manufacture a resin product such as a plastic lens which is strictly required to have a high precision in figure dimension, a high evenness in internal quality and have not any internal distortion.
Up to now, as a conventional metallic mold to mold a transparent component with a relatively large thickness without the internal distortion or a shrink mark by the injection molding, the metallic mold disclosed by Japanese Patent Open to Public inspection Publication No. (J.P OPI Publication No.) 95431/1982 has been very well known. In the metallic mold, a high temperature heating medium and a low temperature heating medium are selectively changed so as to flow into a flow passage of the metallic mold and the flow amount can be controlled by a flow amount regulating valve so that the temperature of the metallic mold can be changed in accordance with a presetting program.
Further, as an injection molding method of producing a high precision plastic lens or a plastic lens which has not any internal distortion and is superior in optical performance, the following methods have been disclosed by J.P OPI Publication No. 11619/1987, 39224/1991 and 132323/1991.
In a method, a metallic mold is heated higher than a glass transition temperature of a resin, a melted resin is introduced into a cavity of the heated metallic mold, a keeping pressure of 400 to 1200 Kgf/cm.sup.2 is applied for 0.1 to 20 seconds, and then, after the gate is mechanically sealed, the metallic mold is gradually cooled.
In another method, the cavity forming surface of a metallic mold is heated up to a temperature at which a resin flows, the cavity is filled with the melted resin, and then the metallic mold is cooled.
Still further, as a manufacturing method for injection molding products whose strength is not lowered by a weld line, a method in which a metal surface temperature of a metallic mold at a position on which a weld line is generated is set within a range of a thermal deforming temperature to a flow-stopping temperature and a non-crystallized resin is introduced into a cavity of the metallic mold has been well known by J.P OPI Publication No. 25425/1992.
Even if the metallic mold disclosed in J.P OPI Publication No. 95431/1982 was used, there has been the following problems. That is, it may be not easy to determine through experiences the presetting program for the metallic mold temperature capable of attaining the object for each kind of resin material. Further, even if the metallic mold temperature is controlled in accordance with the same program, if the resin melting temperature or the cylinder pressure in the molding machine is changed, the object is not necessarily attained, resulting in that a shrink mark, a warp, or an internal distortion may take place on a molding product or the undesired projections (flash ridge) become so large to produce a molding product which need additional process in the finishing process.
Further, the method disclosed in J.P OPI Publication Nos. 39224/1991 and 132323/1991 is the substantially same method as the method disclosed in J.P OPI Publication No. 95431/1982 in which the metallic mold temperature is heated in advance to a temperature at which the resin can flow. Therefore, depending on the resin melting temperature or the cylinder pressure in the molding machine, the method is apt to cause the following problems. A shrink mark, a warp, or an internal distortion may take place on a molding product or the undesired projections become so large to produce a molding product which need additional process in the finishing process.
Still further, the method disclosed in J.P OPI Publication No. 25425/1992 is a method using a metallic mold whose cavity surface temperature on a specific portion is heated to a temperature at which a resin is softened, the similar problems as those mentioned above in the method disclosed J.P OPI Publication Nos. 39224/1991 and 132323/1991.
Since the method disclosed in J.P OPI Publication No. 11619/1987 is a method sealing a gate by filling the cavity with a melted resin and by applying a keeping pressure, if a high keeping pressure was not applied, a shrink mark is apt to take place. In order to apply the high keeping pressure, the metallic mold needs the precision and the strength more. In the result, the method is apt to make the metallic mold large in size or high in cost.